Mostly clear night with some thin cloud around (I could, for
example, see a pretty nice halo around the Moon). The sky was still
very light but given that Jupiter was in a position
where I could see it from my back garden I decided to have a quick
session looking at it with the 905.

Jupiter, including a shadow transit

I got Jupiter lined up in the
905 using a low-power eyepiece and then I
dropped in the 6mm eyepiece. Initially
the image was pretty poor, moving around quite a bit and the
visible detail was much less than previous views of the planet
with the same equipment. That said, the 'scope hadn't had any
chance to cool down.

As usual the two main bands were easily visible and there was
obvious variation on the rest of the disc. I could only make out
three moons (later checking showed them to be
Europa, Ganymede and
Callisto). Europa and Callisto were some
distance out, either side of the planet while Ganymede was quite
close to the planet.

Around 21:58 UT, in very steady moments
(the image was rapidly getting better), I started to notice a
very clear, sharp, dark spot near the north pole of Jupiter. My
initial impression was that I was seeing a shadow of one of the
moons (presumably the Ganymede). To be sure that this wasn't
just an artifact I tried changing eyes, rotating the eyepiece
and shifting the 'scope about to place the planet in a different
part of the field of view. Nothing I did changed
the image.

After watching this a bit more I grabbed my sketching box and,
between 22:10 UT and
22:15 UT, I made the following rough
sketch so that I could compare the view I had with some other
source later.

After doing the sketch I popped into the office and fired up
Starry Night to check what I was seeing. It confirmed
that I was seeing a shadow and that it was the shadow of
Ganymede. My first ever Jovian shadow transit!

Around 22:30 UT some thin cloud moved in
such that my view of Jupiter got a lot worse. The amount of
visible detail was reduced and I could no longer see the shadow.
By 22:39 UT the cloud was still hanging
around and the image had deteriorated even more. I also noticed
that the air suddenly felt quite damp and that a thin layer of
dew was forming on everything.

By 22:50 UT the cloud had finally cleared
and the view of Jupiter had improved again (although it didn't
seem to be as crisp as it had earlier on) but I still couldn't
make out the shadow any more. At 22:55 UT,
given that I still couldn't make out the shadow and that I
needed to get to bed soon, I called it a night on the session.

Checking the following day, using the "Jupiter's Moons" tool
on the
Sky
& Telescope website, I've now further confirmed that it
was Ganymede's shadow. The times given for the event are:

This would explain why I was unable to see the shadow
towards the end of the session: it would have been very close to
the edge of the planet's disc and I would probably have been
unable to resolve it that close with the equipment I was using.